You make a false dichotomy between open and paid. It is quite possible to have closed software that is offered gratis. It is also quite possible to have open software that is developed partly or entirely for a fee.
Corion wants some incentive to make guarantees for something he'd otherwise work on in his spare time, when he's not spending his time making a living. I think that's more than fair. He doesn't have to accept the request for guarantees at all. In fact, he doesn't have to fix bugs or provide the module in the first place. That he would consider a guarantee you can purchase from him is going well above and beyond what most authors of any type of software -- open, closed, free, pay, or whatever -- would consider. Most have waivers of warranty and use-as-you-can types of statements surrounding their code like a cocoon.
